1. Field of the Invention
The present invention relates to a kind of charge pump circuit, and more particularly, relates to an integrated charge pump circuit using thin film transistors.
2. Description of the Related Art
In display circuit integrations, DC—DC converters can be integrated into display panels to lower the required supply voltages of system interface control ICs, such that fabrication cost of the control ICs is reduced. For example, fabrication costs and time-to-market of display module can be reduced greatly, by forming driving circuits by thin film transistors (TFTs) and integration into display panels. However, poor TFT characteristics, such as high threshold voltage (Vt) and low mobility, result in lower power efficiency of the DC—DC converters compared to MOSFETs.
Conventional DC—DC converters are classified as switching regulators and charge pump circuits. Charge pump circuits are more suitable for integration than switching regulators because typical switching regulators are RLC circuits, producing harmonic noise. Charge pump circuits can also be divided into Dickson types, as shown in FIGS. 1a and 1b, and switched-capacitor types. However, due to higher threshold voltage (Vt) of TFTs, the Dickson type charge pump circuits have a great transistor switching loss, reducing entire conversion efficiency thereof. Modified Dickson type charge circuits are proposed to improve power conversion efficiency, but are complicated to drive.
FIG. 2 shows a conventional charge pump circuit 10, however, an extra voltage boost circuit 12 is required to provide boosted clock signals to promote gate drive capability of TFTs in the charge pump circuit 14, preventing charge backflow. In the voltage boost circuit 12, for example, a level shifter is required to produce the boosted clock signals according to clock signals and the output voltage Vout from charge pump circuit 14. Consequently, circuit complexity is increased and entire power conversion efficiency is still poor due to the extra voltage boost circuit.